The present invention relates to polyurethanes with a softening temperature above 80.degree. C. which are NCO-free, reactive via masked isocyanates, essentially linear and extensively pre-lengthened, their preparation and their use as powders which can be processed like a thermoplastic and post-cured.
Powders play an important part in many areas of plastic preparation and processing. Due to their fluidity, they are used, e.g., for the preparation of adhesives, coatings or thin films and sheeting.
The use of decorative plastic sheeting in the interiors of motor vehicles is known (R. Pfriender, Kunststoffe 76 (1986), 10, p. 960 et seq.). According to the prior art, PVC/ABS sheeting which is shaped by thermoforming is generally used and this is subsequently backed with foam. Polyurethane sheeting prepared from liquid systems may be processed by the IMC process ("In-Mold-Coating"). However, the production of parts is very expensive and has still barely been mastered (Dr. M. Wachsmann, Kunststoffberater, 10/1987, p. 27-28).
On the other hand the "powder-slush-molding" process permits the production of much more finely structured sheeting which can be undercut and is thus qualitatively more useful. Here, according to the prior art, PVC powder is used which is gelled by agitation in a heated mold at about 200.degree. C. After cooling the mold, the sheeting can be removed.
The disadvantage of the PVC sheeting which is usually backed with PU foam ("PU" is used as an abbreviation for polyurethane), however, is the mutually negative effect of PVC sheeting and the foam backing upon each other. Catalysts or stabilizers may diffuse out of the PU foam into the PVC covering, and at the same time migration of, e.g., plasticizers occurs from the PVC into the PU foam.
From the point of view of recycling, combination systems made from the same type of plastic are desired. Thus, there is intense interest on the part of the automobile industry in uniform materials in which the above-mentioned negative effects between covering and foam backing do not occur and where there is the possibility of simpler recycling. The use of PU top sheeting which is backed with PU foam would therefore be desirable.
The possibility of subsequently granulating and milling thermoplastic polyurethanes (TPU), which are obtained from the melt by extrusion or strip processes, and finally obtaining PU powders is known (German Auslegeschrift 3,916,874). These powders may be processed into sheeting by sintering.
The use of thermoplastic PU systems permits sintering of the powder with the melt flowing to form a homogeneous covering. With pure thermoplastic (linear) systems, however, the phenomenon which occurs is that on sintering, the low viscosity TPU melt flows or drains away to vertical surfaces of the mold or to overheated places in the mold. This leads to variable thickness of the layer or even to holes in the sheeting. However, the processing of thermoplastics presupposes an initially largely linear system. Cross-linking which takes place during the processing procedure would, however, be disadvantageous due to the increase in viscosity which would occur (in the following "cross-linking" includes cross-linking in the sense of the formation of branched structures).
It is known that isocyanates may be reacted with mono-functional reactants ("capping agents") such as, e.g., oximes, caprolactam or phenol derivatives, to give thermoplastic adducts. By using these "capped isocyanates", systems may be obtained which are thermoplastic up to the cleavage temperature of the adduct and cross-linked above this temperature.
Furthermore, the use of uretdiones or polyisocyanates which contain uretdione groups in single-component PU reactive systems is known per se. The polyisocyanates which contain uretdione groups are obtained by the placing together ("dimerization") of two isocyanate groups in the presence of special catalysts. The uretdione groups split up at elevated temperature with the re-formation of two isocyanate groups which, e.g. in the presence of OH groups, react further to produce cross-links. This happens without the release of a "capping agent". See D. Dieterich in "Methoden der organischen Chemie" (Houben-Weyl), E 20, p. 1650ff, New York 1987.
When producing thermoplastic PU systems by conventional extrusion or strip processes in the melt, as is described, e.g., in German Auslegeschrift 3,916,874, sufficiently high temperatures must be used to keep the melt viscosity low. This may cause problems when producing the above-mentioned one component PU systems which contain capped isocyanates because the cleavage temperature may be achieved or exceeded and the system may become noticeably cross-linked during preparation.
The preparation of one component PU coating powders is well-known and is the prior art. These are prepared by extruding mixtures of polyester resins or acrylate copolymers which contain functional hydroxyl groups and masked isocyanates at between 80.degree. and 130.degree. C. and then finely milling them (see Becker/Braun, Kunststoff-Handbuch, vol. 7, "Polyurethane", Carl Hanser Verlag, Munich, Vienna, 1983; DE-A 2 707 656). In order to ensure the storage stability of coating powders and to prevent caking at temperatures up to 50.degree. C., the glass transition range of the component which reacts with NCO groups, e.g. the polyester resin with functional OH groups, must be relatively high, e.g., well above 0.degree. C. Polyesters which impart flexible properties can be used only in small amounts due to their low glass transition temperature.
In the case of coating powders, therefore, in principle no increase in molecular weight or cross-linking, and therefore no pre-lengthening, takes place during preparation. However, in coating powders the component which reacts with NCO groups is present in an essentially free state alongside the completely masked isocyanate component. Only during processing (fusion) does unmasking and cross-linking take place, with an increase in molecular weight and the formation of polyurethane.
Accordingly relatively long processing times are required, e.g., 10 to 30 minutes at 160.degree. to 200.degree. C. For an application such as the "slush-molding" process mentioned above, however, it would be an advantage if an already extensively pre-lengthened, but still thermoplastic polyurethane could be used as the powder. This might be modified by the cross-linking occurring during processing, leading to an increase in viscosity.
The object of the present invention was, thus, to obtain a reactive, extensively pre-lengthened polyurethane, which is processable to give a storage-stable, pourable powder and which is useable as a powder in the "slush-molding" process, by developing a method which does not involve the risk of premature cross-linkage under the conditions of preparation.